Bone conduction is the principle of transmitting vibrations via the skull bone to the inner ear, i.e. the cochlea. Several different types of bone conduction hearing aids are available. The two most common types of such hearing aids are the traditional bone conductors and the direct bone conductors. The traditional bone conductor consists of a hearing aid with a vibrator that is pressed against the skull. The vibrations are then transmitted through the skin and the skull bone into the cochlea. The direct-bone conductors include an anchoring component that is attached to the skull bone and through which the vibrations are directly transmitted to the skull bone.
All of such devices require a vibrator where the electrical signal is converted and transferred into mechanical vibrations. Preferably, electromagnetic vibrators are used in these applications. There are several drawbacks with a traditional non-balanced variable reluctance vibrator. One drawback is that there is a static strong magnetic force biasing to collapse the air gap. The strong static magnetic flux is required as a biasing flux to get a sufficient linearity of the vibrator. To prevent the magnetic force from collapsing the air gap the suspension spring constantly has to counteract the magnetic force. Due to the constantly high load on the suspension spring there is a significant risk for a collapse of the air gap. If a stiffer spring is used, the low frequency response of the vibrator is reduced, which is not desirable. The low frequency response may in that case be improved by adding mass to the vibrator, however this makes it bigger which is not desirable. This design also presents a built-in distortion component that reduces the sound quality of the vibrator.
In some suggested designs, the armature is balanced so that the static force on the suspension elements is minimized and the static and the dynamic magnetic fluxes go partly separated through the magnetic circuit. This may lead to some advantages in the magnetic circuit, however it leads to a complex design that is expensive to manufacture.
In some designs more than two air gaps are required or there has to be at least two magnets in the design. Since each air gap need to be quite small to get an efficient design each air gap requires high precision of the mechanical and magnetic components, and it is therefore a significant drawback to have more air gaps than necessary. By using air gaps with magnetic forces perpendicular to the axial direction in the vibrator there is a risk of the air gap collapsing also in this direction in addition to the risk of the air gap collapsing in the axial direction. Small high precision magnets as those required for this kind of vibrators are costly both to purchase and to handle and makes the design complex. It is therefore desirable to keep down the number of magnets. Also the coils are difficult to handle and it is also desirable to keep down the number of coils.